1. Field of the Invention
The present invention relates to a communication apparatus which enables coexistence of systems, and more particularly to a communication system which enables an access system which is used in common by a plurality of users and in-home systems which are used separately by the respective users to coexist on the same communication medium, such as a power line or the like.
2. Description of the Background Art
Power Line Communications (PLC) has attracted attention as a technology for connection of a network apparatus, such as a broadband router or the like, so as to access from a Personal Computer (PC) or the like in a home to the Internet. In the power line communication, since an existing power line is used as a communication medium, it is not necessary to construct a new infrastructure, and high-speed communication can be achieved only by inserting an AC mains plug into an AC mains outlet in a home. Therefore, research and development, and demonstration experiments have been vigorously conducted all over the world, and in Europe and the USA, and a number of PLC projects have already been commercialized.
FIG. 18 is a diagram illustrating a general configuration when a PC is used to access from a home to the Internet.
A PC 201 which is used by a user in a home is connected via an Ethernet 211 to a broadband router 202, through which the PC 201 is connected via an access line 212 to the Internet 222. As the access line 212, ADSL (Asymmetric Digital Subscriber Line), FTTH (Fiber To The Home) or the like is generally used. Here, it is often that a place where the access line 212 is withdrawn into the home is different from a room where the PC 201 is placed. In this case, a cable of the Ethernet 211 needs to be extended from the broadband router 202 to the PC 201.
In the field of power line communication, in order to reduce the extension, a conversion adaptor (hereinafter referred to as a P/E conversion adaptor) of power line communication and Ethernet has been commercialized. FIG. 19 illustrates a general configuration related to access to the Internet when the P/E conversion adaptor is used.
A PC 301 which is used by a user in a home is connected via an Ethernet 311 to a P/E conversion adaptor 303, through which the PC 301 is connected via an outlet to an in-home power line 314. Data is transferred to a P/E conversion adaptor 304 for a broadband router 302 by power line communication. The P/E conversion adaptor 304 is connected via an Ethernet 313 to the broadband router 302. The broadband router 302 is connected via an access line 312 to the Internet 322.
Also, a technology for utilizing power line communication as an access system has been developed. FIG. 20 is a diagram illustrating an exemplary configuration where a high-speed power line technology is used for both an in-home system and an access system when the Internet is accessed from a PC in a home. Here, a PC 401 which is used by a user in a home is assumed to have an in-home-system high-speed power line communication function.
An in-home system employing power line communication refers to a communication network which is composed of an in-home power line 411 (transmission medium) and an apparatus connected to a power line outlet provided in a home (including an outer wall). A logical configuration range of the in-home system is the inside of a house, and is used for AV streaming, audio transmission, broadband internet access, home control, and the like.
On the other hand, an access system employing power line communication refers to a communication network which is composed of a low-voltage power distribution line 412 from home to a pole transformer provided on an outdoor utility pole and an in-home power line 411 (transmission media), and an in-home apparatus 403 provide in a home and an apparatus provided outdoors, and in some cases, may include an intermediate-voltage power distribution line 413 from the pole transformer to an electric power substation. A configuration range of the access system ranges from the in-home apparatus 403 to a connection point of the Internet, and is used for a broadband internet service, an audio service (VOIP), a video service (IPTV, IP broadcasting), and the like.
When an access system employing power line communication is applied to a building or an apartment house, an in-home apparatus provided in each room of the building or each apartment of the apartment house and a power distribution line in the building or the apartment house constitute the access system.
The PC 401 performs data communication with a router 402 having an in-home-system high-speed power line communication function, via an AC mains outlet connected to the in-home power line 411. The router 402 is connected via an Ethernet 414 to an in-home apparatus 403 having an access-system high-speed power line communication function. The in-home apparatus 403 communicates with an access-system high-speed power line communication apparatus provided in a pole transformer 404 on a utility pole, via the low-voltage power distribution line 412 extended from the inside to the outside of the house. The Internet is accessed from the pole transformer 404, using an optical fiber or the like.
In this case, the same power line is used in communication between the PC 401 and the router 402, and communication between the access-system in-home apparatus 403 and the access-system communication apparatus included in the pole transformer 404. In this case, if both the communications are based on the same communication scheme, media access in which a communication medium is used in common by means of Time Division Multiplexing (TDM) or Frequency Division Multiplexing (FDM) is easily achieved by exchanging a predetermined signal. However, a variety of power line communication schemes have already been actually developed, and a plurality of schemes are considered to be introduced in the future. Therefore, it cannot be necessarily expected that a power line communication scheme which is used in in-home-system communication and a power line communication scheme which is used in access-system communication are based on the same technology. Also, in in-home-system communication, it is highly likely that power line communication based on a plurality of different techniques coexist. However, for communication apparatuses which cannot communicate with each other, it is difficult to allocate the resource of a communication medium in a temporal or frequency-spatial manner.
To solve such a problem, it is considered that a coexistence signal which has a simple configuration is defined, and communication apparatuses employing various power line communication schemes are enabled to transmit and receive the coexistence signal, thereby allocating the resource of a power line communication medium in a temporal or frequency-spatial manner.
For example, in Patent Document 1 (Japanese Patent Laid-Open Publication No. 2000-151547), a communication terminal employing an OFDM technique transmits and receives a signal including a frame synchronization signal using only a specific subcarrier. The communication terminal uses the signal to establish synchronization between an access-system base station and an in-home-system base station, and thereafter, the access-system base station allocates a time slot to the in-home system. Thereby, it is possible that, while the access system and the in-home system coexist in the time domain, the access system which is expected to be used in common to a plurality of homes, uses a power line communication medium with a higher priority than that of the in-home system.
FIG. 21 is a diagram illustrating a frame structure employed in Patent Document 1. In Patent Document 1, communication is performed by repeating the frame of FIG. 21. In FIG. 21, a synchronization slot 501 is used to establish synchronization between an access system and an in-home system, and includes a specific synchronization signal. Thereby, the access system and the in-home system can be synchronized with each other in units of a frame. Next, a control slot 502 is used to inform how the following data slot 503 is used by the access system and the in-home system. A base station of the access system uses this slot to inform each base station of the in-home system of an available slot. With the above-described mechanism, the coexistence in the time domain of the access system and the in-home system is secured.
However, such coexistence due to TDM is disadvantageous with respect to communication for which QoS (Quality of Service) needs to be taken into consideration.
The QoS of communication is represented by a data transmission rate, a tolerable maximum transmission delay, or the like. As general applications, there are AV stream transmission which can relatively tolerate a delay, though a high data transmission rate needs to be secured, and two-way audio data transmission which has a strict requirement for limitation on delay, though a data transmission rate is low. Further, an application, such as a high-definition video telephone or the like, is considered which needs to simultaneously satisfy a relatively high transmission rate and a strict requirement for limitation on delay.
Here, a general frame structure in digital data communication is illustrated in FIG. 22. Digital data communication is generally performed in units of a frame of FIG. 22. The frame can be divided into a header 601 which uses a modulation technique which is relatively robust against noise (i.e., a low transmission rate), a payload 602 which is an area for carrying user data or the like, and an error detection/correction code 603 which is used by a reception apparatus to detect whether or not the payload 602 has been contaminated with a transmission error during transmission, or correct the transmission error. Here, the header 601 has a fixed length, and in this portion, a fixed modulation technique having a low transmission rate is often used. The payload 602 often has a variable length, and a modulation technique used therein and a size thereof are generally described in the header 601. The error detection/correction code 603 has a limitation on an amount of error which can be detected/corrected, and if the number of error bits occurring in the payload 602 is within a range which enables error detection/correction, detection/correction can be achieved.
In such digital data transmission, since the header 601 has a fixed length and the payload 602 has a variable length, the transmission efficiency increases (i.e., a larger user data which can be transmitted within a predetermined time) with an increase in the proportion of the payload 602 occupying a frame. Conversely, as the payload 602 is increased, the amount of transmission errors occurring in the payload 602 increases, and therefore, when the payload 602 is received by a reception apparatus, the possibility increases that the amount of transmission errors occurring in the payload 602 exceeds the detection/correction capability of the error detection/correction code 603. For digital data transmission, it is important to establish a balance between the large payload and the transmission error so as to achieve maximum data transmission efficiency.
In view of the above description, it is preferable that a frame having a relatively large payload be used for AV stream transmission requiring a high data transmission rate, and a frame having a small payload be used for two-way audio data transmission requiring a low data transmission rate.
It is here assumed that there are two data streams which have QoS requirements of FIG. 23, i.e., an AV stream which has a data transmission rate of 24 Mbps and tolerates a maximum delay of 300 msec, and a two-way stream having two pieces of audio data which have a data transmission rate of 64 kbps and tolerate a maximum delay of as small as 10 msec.
It is also assumed that, in an environment where the access-system power line communication and the in-home-system power line communication coexist as illustrated in FIG. 20, the AV stream is communicated from the in-home PC 401 to a television 405, and the two-way stream is communicated between a friend's house and an in-home IP telephone 406 via the Internet. In this case, according to the technology of Patent Document 1, in the data slot 503 of the frame of FIG. 21, a sufficient space for transmission of the two-way stream needs to be allocated for the access system, and a sufficient space for transmission of the AV stream and the two-way stream needs to be allocated for the in-home system. Here, since there is a limitation (a maximum delay of 10 msec) on the two-way stream, a slot having a maximum interval of 10 msec needs to be allocated for both the access system and the in-home system which transmit the two-way stream. To achieve this, the length of the frame of FIG. 21 needs to be 10 msec or less, or a plurality of slots need to be allocated in a manner which avoids an interval of 10 msec or more in the data slot 503 of the frame. However, when the former is achieved, the frame length becomes shorter, so that the transmission efficiency of an AV stream requiring a high data transmission rate decreases. In this case, even when a two-way stream is not present, the transmission efficiency of the system cannot be increased. Also, when an attempt is made to use the latter to satisfy the QoS requirement of a two-way stream, the amount of information to be transmitted in the control slot 502 increases.
As described above, it is difficult to simultaneously achieve a high data rate and a small delay time in the case of coexistence due to TDM.
A coexistence control employing FDM is preferable in terms of satisfaction of different QoS requirements of a plurality of types of data streams. If FDM is used, each data stream can occupy a specific frequency band, and therefore, a frame can be transmitted at any arbitrary time using an allocated frequency.
When systems having communication schemes different from each other are caused to coexist using FDM, it is difficult to perform a complicated control in a coexistence mechanism which cannot use a limited band. This is because, although coexistence of systems having communication schemes different from each other requires a signal common to all the systems, it is not possible to allocate a large amount of frequencies or time to the common signal, in terms of data communication efficiency. Therefore, a method is generally adopted in which fixed frequency channels are prepared and systems use the respective channels.
Here, a frequency used by an access communication system varies among the set price zones of services, the scales of systems, or target services. Therefore, when an access communication system and an in-home communication system coexist due to FDM, a maximum frequency band for the access communication system needs to be secured in the case of fixed frequency allocation, taking into consideration the case where the access communication system is a system which requires a broad frequency band. Actually, free space occurs in the frequency band secured by the access communication system, or the like, i.e., there is a possibility that frequency efficiency is deteriorated. On the other hand, in the in-home communication system, it is desirable to use as large a number of bands as possible in order to improve or maintain quality, however, in the case of fixed frequency allocation, a free frequency band of the access communication system cannot be utilized.